Astronomers Michael Belton and Harvey Butcher have made an important observation. They didn't see Halley's comet.
Their failure to find it when looking with the Kitt Peak National Observatory four-meter telescope last Dec. 2 and 3 has helped them to estimate the size, mass, and brightness of the famous object.
As they explained recently in Nature, these estimates suggest that the comet may be so faint that astronomers will not recover it until early in November 1984. But they also suggest that the comet could be bright enough to begin to be seen late this September or early in October.
Picking the most probable figures from the range of their estimates, Belton and Butcher say Halley's comet is about 4 kilometers across with a mass of 34 trillion kilograms (37.5 tons). It is quite faint, having a magnitude no less than 24.3 on the astronomer's photometric (V band) scale. On this scale, larger magnitudes mean fainter objects.
The comet, at this stage, is hard to find under the best conditions. It moves very fast along a path that lies almost in the plane of our galaxy, where there are many background stars to confuse observers. And, of course, it is faint. The Kitt Peak astronomers say they are confident they would have seen it, however, if it were above the observing threshold of their equipment.
To begin with, they are ''almost certain'' the comet was within their field of view. They are using an observing guide prepared for the International Halley Watch by Donald K. Yeomans of the NASA-Jet Propulsion Laboratory. Yeomans has said he believes the positions he gives for the comet are accurate to about 10 seconds of arc. That's well within the 60-by-60-second field of view of the Kitt Peak observations.
There is a possibility that the comet is hidden by an image of a star in the photos. Belton and Butcher put the probability of this happening at about 28 in 100, meaning this would be expected in about 28 out of 100 observations. Calling this possibility ''sizable but not hopeless,'' they say they think they have avoided it by taking care in the timing of their observations.
Using a general knowledge of comets and standard assumptions about their composition and orbital dynamics, the two astronomers then estimate how big Halley's comet could be and still not have been seen last December. They consider two cases -- the comet has a surface of relatively fresh ice or it has an old, fairly dark icy surface.
Of course, it remains to be seen how close they have come to the right size values. This will be evident when astronomers do finally detect the comet as it heads for its rendezvous with the sun in 1986.
We may not have so long to wait for ''this moment of truth.'' The Kitt Peak astronomers note that the comet may have been only half a magnitude below their observing threshold in December. If this is the case, then they could see it early this fall.